Ballast instant start circuit

Details Ballast instant start circuit Ballast instant start circuit

2000-01-28

2001-05-22

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S219000, C315S224000

Reexamination Certificate

active

06236168

ABSTRACT:

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
FIELD OF THE INVENTION
The present invention relates generally to circuits for driving a load and more particularly to a ballast circuit for energizing one or more lamps.
BACKGROUND OF THE INVENTION
As is known in the art, there are many of types of artificial light sources. Exemplary sources of artificial light include incandescent, fluorescent, and high-intensity discharge (HID) light sources such as mercury vapor, metal hallide, high-pressure sodium and low-pressure sodium light sources.
Fluorescent and HID light sources or lamps are generally driven with a ballast which includes various inductive, capacitive and resistive elements. The ballast circuit provides a predetermined level of current to the lamp for proper lamp operation. The ballast circuit may also provide initial voltage and current levels that differ from operational levels. For example, in so-called rapid start applications, the ballast heats the cathode of the lamp with a predetermined current flow prior to providing a strike voltage to the lamp. Thereafter, the ballast provides operational levels of voltage and current to the lamp thereby causing the lamp to emit visible light.
One type of ballast circuit is a magnetic or inductive ballast. One problem associated with magnetic ballasts is the relatively low operational frequency which results in a relatively inefficient lighting system. Magnetic ballasts also incur substantial heat losses thereby further reducing the lighting efficiency. Another drawback associated with magnetic ballasts is the relatively large size of the inductive elements.
To overcome the low efficiency associated with magnetic ballasts, various attempts have been made to replace magnetic ballasts with electronic ballasts. Electronic ballasts energize the lamps with a relatively high frequency signal and provide strike voltages for instant-start lamp operation.
One type of electronic ballast includes inductive and capacitive elements coupled to a lamp. The ballast provides voltage and current signals having a frequency corresponding to a resonant frequency of the ballast-lamp circuit. As known to one of ordinary skill in the art, the various resistive, inductive and capacitive circuit elements determine the resonant frequency of the circuit. Such circuits generally have a half bridge or full bridge configuration that includes switching elements for controlling operation of the circuit.
An electronic ballast may operate in a start-up mode known as instant-start operation. In instant-start mode, the ballast provides a voltage level sufficient to initiate current flow through the lamp to cause the lamp to emit light, i.e., a strike voltage. An exemplary strike voltage is about 500 volts RMS. After application of the strike voltage, the ballast provides an operational voltage level, e.g., 140 volts RMS to the lamp.
Where a ballast energizes a plurality of lamps, the lamps are preferably coupled to the ballast such that each lamp operates independently. With this approach, failure or removal of one lamp does not affect other lamps. In addition to independent operation of each of the lamps, the ballast circuit should also provide a strike voltage to lamp terminals from which a lamp has been removed. A steady state strike voltage at the lamp terminals causes a lamp to emit light when the lamp is placed in contact with the lamp terminals.
In one known circuit arrangement, an output isolation transformer is used for energizing one or more lamps. A series-coupled first lamp and first buffer capacitor are coupled across a winding of the isolation transformer. Additional series-coupled lamps and buffer capacitors can be coupled across the transformer. The transformer provides a strike voltage, such as about 500 volts, across the series-coupled lamps and buffer capacitors to light the lamps as they are placed in circuit. When current begins to flow through the lamps, however, the voltage across the lamps drops to an operational level, 140 volts for example. The remainder of the 500 volts appears across the buffer capacitor resulting in relatively inefficient circuit operation. To provide a steady state strike voltage at the lamp terminals, a relatively large transformer is required. As understood to one of ordinary skill in the art, the large transformer generates significant heat that must be dissipated to prevent overheating of the circuit. Thus, the isolation transformer can be a significant factor in the overall size and cost of the ballast circuit.
It would be desirable to provide a relatively compact and low cost ballast circuit that provides independent operation and instant-start voltages to each of a plurality of lamps or other loads driven by the ballast circuit.
SUMMARY OF THE INVENTION
The present invention provides a circuit for energizing a plurality of loads and for providing strike voltages for instant-start operation. Although the circuit is primarily shown and described as a ballast circuit for energizing lamps, and in particular fluorescent lamps, it is understood that the invention finds application with a variety of different circuits and loads.
In one embodiment of the invention, a ballast circuit for energizing a plurality of lamps includes a resonant circuit, such as an inverter circuit in a half-bridge configuration. The resonant circuit includes inductively coupled first and second inductive elements connected to respective first and second lamp terminals. In an exemplary embodiment, the first and second inductive elements are formed from corresponding first and second windings formed on a single bobbin. The resonant circuit further includes a first resonant capacitive element coupling the first and second inductive elements. This arrangement allows the inductively coupled first and second inductive elements to operate as independent inductive elements. The circuit also provides a strike voltage across lamp terminals from which a lamp has been removed for instant start operation. The strike level voltage appears across the lamp terminals due to resonance between the inductive and capacitive circuit elements.
Independent operation of the inductively coupled first and second inductive elements is achieved by eliminating induced current flows in the first and second inductive elements. Without induced current flow, the first and second inductive elements are not coupled to each other and thus can operate independently of each other. While the first and second lamps are being energized, there is substantially equal current flow through each of the inductive elements to the respective lamps. When one of the lamps, such as the first lamp, is removed from the circuit the first capacitive element begins to resonate with the first and second inductive elements. The impedance value of the first capacitive element is selected such that the first capacitive element resonates with the inductive elements at a frequency at or near a resonant frequency of the overall inverter circuit. As is known to one of ordinary skill in the art, the resonant frequency of the overall circuit is determined by the impedances of the various resistive, inductive and capacitive circuit elements. As is also known, current does not flow through a parallel resonant inductive/capacitive (L-C) circuit at the resonant frequency of the L-C circuit. Thus, in this circuit arrangement, there is no induced current flow between the first and second inductive elements, i.e., they are independent. Resonance of the circuit elements generates a voltage level at the first lamp terminals that is sufficient to strike a lamp as it is placed in circuit thereby providing instant start operation.
In another embodiment in accordance with the present invention, a circuit has first and second circuit paths coupled to respective first and second lamp terminals. The circuit paths extend from a point between first and second switching elements, which are coupled in a half-bridge configuration. The first circuit path includes a first inductive element, a first DC-blocking

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